In television receivers, especially projection type television receivers, management of the black level of the reproduced image is critical to maintaining perceived detail and sharpness in dark image portions. Unfortunately, the electro-optic characteristics of the CRT itself, and in the case of projection type television receiver, the optical system itself, interferes with the faithful reproduction of dark image portion. This is primarily due to the non-linearity or "gamma" of the CRT, and also to light scattering effects, especially in a projection television receiver.
Gamma refers to the non-linear voltage drive to light output of the CRT. When the television transmission standard was originally defined, television cameras were designed to have a complimentary gamma function to that of CRTs so that the overall transmission system from camera light input to CRT light output was linear. However, as time went on, CRTs were designed for more and more light output, resulting in a in a different (higher) gamma than the one of earlier CRTs. Cameras, however continue to conform more or less to the original defined gamma characteristics for television cameras. Thus, the modern television receivers exhibit a non-linear light output characteristic relative to the original scene scanned by the television camera. The result of this mis-match in gammas between the camera and the CRT is that low level luminance scenes are displayed with less contrast than the original scene and high level luminance scenes with more contrast than the original scene.
Unnaturally high contrast areas of the picture are rarely complained about. However, there may be a loss of perceived detail and sharpness in low light areas having unnaturally low contrast. This situation could be corrected by adding gamma correction circuitry to the television receiver to bring the effective gamma of the television receiver back closer to the one contemplated by the original television transmission standard. However, this is rarely done because of circuit complexity and cost. In addition, gamma correction tends to enhance noise in the low light areas of the reproduced image relative to an arrangement without gamma correction. This results because noise introduced in the transmission path is amplified by the gamma corrector.
The second phenomena which adversely affects perceived low light area image detail and sharpness, light-scattering, is due to light from high light areas spilling into all areas causing black areas to appear less black. With images having a significant high light image portion i.e., images with a so called high average picture level (APL), light scattering may result in loss of detail in dark areas. Light scattering is typically a more severe problem in a projection television systems due to their optics.